1. Field of the Invention
The present invention generally relates to digital cellular mobile communications systems and, more particularly, to a scheme for .pi./4 phase shift differential encoding which reduces the complexity of the encoder.
2. Description of the Prior Art
Digital cellular mobile communications present many design challenges for the realization of a practical and commercially viable system. On the one hand, a considerable amount of data, both voice and control, must be transmitted between a base station and a number of mobile stations. The amount of data needed for the system is dictated in part on the quality of the voice transmission required for the system and the control functions implemented by the system. On the other hand, the mobile stations need to be both inexpensive and lightweight, and this dictates, among other things, the use of a nonlinear transmitter amplifier. Although weight is typically not a factor in the base station, expense is, making a nonlinear transmitter amplifier desirable there as well.
Differential or quadrature phase shift keying (QPSK) is a popular technique for transmitting large amounts of data within a limited bandwidth. However, the use of nonlinear amplifiers in a QPSK system causes the transmitted spectrum to exceed the bandwidth assigned to a transmission channel. A solution to this problem is to use .pi./4 differential phase encoding which exhibits much less spread in spectrum than QPSK as a result of nonlinear amplification and also differential demodulation is simpler. Unfortunately, proposed techniques for implementing .pi./4 differential phase shift encoding are both complex and expensive, offsetting any economies realized by the use of nonlinear amplifiers.